On the mechanism of Lewis base catalyzed aldol addition reactions: Kinetic and spectroscopic investigations using rapid-injection NMR

The mechanistic foundations of the Lewis base catalyzed aldol addition reactions have been investigated. From a combination of low-temperature spectroscopic studies (²⁹Si and ³¹P NMR) and kinetic analyses using a rapid-injection NMR apparatus (RINMR), a correlation of the ground states and transition structures for the aldolization reactions has been formulated. The aldol addition of the tert-butylsilyl ketene acetal of tert-butyl propanoate with 1-naphthaldehyde is efficiently catalyzed by a combination of silicon tetrachloride and chiral phosphoramide Lewis bases. The rates and selectivities of the aldol additions are highly dependent on the structure of the Lewis bases: bisphosphoramides give the highest rate and selectivity, whereas a related monophosphoramide reacts slowly and with low selectivity. The monophosphoramide shows no nonlinear behavior. All of the additions show a first-order kinetic dependence on silyl ketene acetal and 1-naphthaldehyde and a zeroth-order dependence on silicon tetrachloride. The kinetic order in catalyst is structure dependent and is either half-, two-thirds-, or first-order. All of the phosphoramides are saturated with silicon tetrachloride in some form, and the resting-state species are mixtures of monomeric and dimeric, pentacoordinate cationic, or hexacoordinate neutral complexes. These data allow the formulation of a unified mechanistic scheme based on the postulate of a common reactive intermediate for all catalysts.